1. Field of the Invention
The present invention relates generally to wireless communication systems and more particularly to an improvement in the range of a wireless LAN device.
2. Description of the Related Art
A wireless communication device in a communication system communicates directly or indirectly with other wireless communication devices. For direct/point-to-point communications, the participating wireless communication devices tune their receivers and transmitters to the same channel(s) and communicate over those channels. For indirect wireless communications, each wireless communication device communicates directly with an associated base station and/or access point via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or access points communicate with each other directly, via a system controller, the public switch telephone network, the Internet, and/or some other wide area network.
Each wireless communication device participating in wireless communications includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver. Typically, the transmitter includes one antenna for transmitting radiofrequency (RF) signals, which are received by one or more antennas of the receiver. When the receiver includes two or more antennas, the receiver selects one of antennas to receive the incoming RF signals. This type of wireless communication between the transmitter and receiver is known as a single-output-single-input (SISO) communication.
Different wireless devices in a wireless communication system may be complaint with different standards or different variations of the same standard. For example, 802.11a an extension of the 802.11 standard, provides up to 54 Mbps in the 5 GHz band and uses an orthogonal frequency division multiplexing (OFDM) encoding scheme. 802.11b, another extension of the 802.11 standard, provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11g, another extension of the 802.11 standard, provides 20+ Mbps in the 2.4 GHz band and also uses the OFDM encoding scheme. 802.11n, a new extension of 802.11, is being developed to address, among other thins, higher throughput and compatibility issues. An 802.11a complaint communications device may reside in the same WLAN as a device that is complaint with another 802.11 standard. When devices that are complaint with multiple versions of the 802.11 standard are in the same WLAN, the devices that are complaint with older versions are considered to be legacy devices. To ensure backward compatibility with legacy devices, specific mechanisms must be employed to insure that the legacy devices know when a device that is complaint with a newer version of the standard is using a wireless channel to avoid a collision.
Currently, most SISO WLANs are IEEE 802.11 complaint. A current communications system provides a range extension on a SISO system by taking an 802.11a/802.11g signal and cutting the symbol rate. Specifically, the current communications system achieves range extension by dividing a symbol clock by 24, i.e., the inverse of Super-G, which doubles the clock frequency. When the symbol clock is divided, the maximum symbol duration is 96 usec. and the corresponding rate is 250 kbps. For example, the current communications system takes an 802.11a/802.11g signal that is 16.5 MHz, divides the symbol clock by 24 and cuts the signal to 687.5 kHz. When the bandwidth for a signal is reduced, the integrated thermal noise density of the receiver is also reduced. Therefore, when the bandwidth is reduced by a factor of 24, the thermal noise floor is decreased by 10*log 10(24). This results in a 16 DB “gain” in the sensitivity of the receiver which is equivalent to at least 3 times improvement in the range of a typical wireless system. The cost of this implementation, however, is that the data rate is also decreased by a factor of 24. Furthermore, since legacy systems in the same cell as the current communications system may not detect this very narrow bandwidth, the current communications system does not interoperate with legacy 802.11a/802.11g systems in the same cell. Specifically, a legacy 802.11a/802.11g device may not detect overlapping Base Service Set (BSS) transmissions from the current system and as such the legacy 802.11a/802.11g system will not set its Clear Channel Assessment (CCA) bits appropriately. Therefore, in dense deployments, such as apartment buildings, network chaos is likely to occur when an active BSS in the current communications system overlaps with an active legacy BSS transmission.